A number of fan draft hybrid cooling towers have been developed, the main target of which is visible plume abatement at least in the period of most susceptible for fog formation. In fact, it means practically a primary wet cooling system with an added secondary dry cooled section (approx. with 10% of heat duty). Such hybrid cooling towers are available for smaller or medium heat rejection requirements, and are of individual cell-type or have a row of such cells arranged in a rectangular shape. An example of the cell-type hybrid cooling towers is disclosed in U.S. Pat. No. 3,923,935. A further type of hybrid cooling towers have a round shape and are generally used for large cooling duties—corresponding technical solutions are disclosed e.g. in U.S. Pat. Nos. 6,499,728 B2 and 8,622,372 B2. The latter type of hybrid cooling towers served the Neckarwestheim NPP (nuclear power plant) in Germany and also has been serving the Moorburg coal fired power plant in Hamburg, Germany.
The main object of all these systems is to mix the oversaturated airflow from the wet cooled section with dry airflow from the added dry cooled section to achieve an acceptable reduction of visible plume in winter period.
A major drawback of the prior art hybrid cooling towers is that they necessitate the use of special built-in mixing structures above the wet fills for mixing the dry and humid air strings. Such mixing structures not only represent extra investment cost and reduce the available tower height for inducing natural draft, but they impose a significant pressure loss on the cooling air, thus increasing auxiliary power requirements.
A further bunch of drawbacks originates from the arrangement of the wet fills and their wet fans. Wet fills are located practically all over the tower base, the arrangement extending to the periphery of the tower shell and the forced draft wet fans ar arranged externally with respect of the wet fills. To reduce high fan and hydraulic noises resulted by the arrangement needs sophisticated and costly noise attenuation and leading to further increase of auxiliary power need.
A general disadvantage of the prior art hybrid towers is that their arrangement does not support flexible operation which is required to achieve notable water saving.
There were some further suggestions for mechanical draft cooling towers to create a vortex by applying tangential and downward cooling air inlet, aiming at reducing the fan power or even producing surplus power; see e.g. U.S. Pat. Nos. 4,157,368 and 4,499,034. However, such systems are not adapted/suitable for the known fan draft hybrid cooling towers.